Resistance spot welding may be used to join metal layers. In general, resistance welding is a joining process in which metal may coalesce at faying interfaces between the metal layers via resistance heating from the passage of electric current under applied force.
In particular, resistance spot welding may be used to join three metal layers so that a middle metal layer is sandwiched between two outer metal layers. For such workpieces, certain industrial welding specifications often require that a thickness ratio between the two outer metal layers should not be more than 1:2, and that a thickness ratio between the middle metal layer and any of the two outer metal layers should not be more than 1:3. Uniform heat distribution between each faying interface generally enhances weld quality and strength for such workpieces.